Pathogens, sediments and nutrients: the nasties making our rivers unsafe

This was first published in the Listener, 31 July 2014.

When I was a kid, in the 1970s, the only “unsafe” water I was aware of was the geothermal hot pools of the Taupo Volcanic Zone. If you put your head under the water, I believed, an amoeba would swim into your ear and eat your brain. Or something like that.

I was happier swimming in cold water. On South Island holidays, we’d swim in freezing glacier-fed lakes and rivers. Sometimes, closer to home, we’d free camp on Wairarapa farmland and swim in the local rivers.

But I don’t think I’d take my children to some of the same swimming spots – especially the ones on farmland. In July 2013, the Ministry for the Environment reported that 61% of the river sites it monitored were unsafe for swimming and should be avoided. Annual reports on recreational water quality have now been dropped, so it’s hard to know if things are getting better or worse.

What makes our fresh water “unsafe for swimming”? In a 2012 report on the science of water quality, the Parliamentary Commissioner for the Environment said the three main water pollutants of greatest concern to our fresh water – rivers, streams, lakes, wetlands and estuaries – were pathogens, sediments and nutrients.

If you’re swimming, the biggest risk to your health comes from pathogens – bacteria or viruses – that can enter your body through your mouth, nose or ears. Pathogens can enter waterways through contamination from human sewage or animal manure; for example, from effluent run-off from farmland, human wastewater discharges, stormwater outfalls and domestic- and wild-animal waste.

Pathogen levels are often highest after rainfall when faecal matter is carried from the land into waterways. According to the environment ministry’s 2013 Suitability for Swimming indicator update, the potential human health effects of these pathogens are numerous, although mostly “minor and short-lived”.

The list includes “gastro-intestinal illnesses with symptoms like diarrhoea or vomiting, and infections of the eye, ear, nose and throat. However, there are other potentially more harmful diseases such as giardiasis, cryptosporidiosis, campylobacteriosis and salmonellosis. Hepatitis A can also be contracted from contaminants in the water and can lead to long-term health problems.”

Nutrients such as nitrogen and phosphorus are useful on land, where they can stimulate plant growth. But in water they can cause excessive growth of weeds, slime and algae and can be toxic – including for humans – at very high levels.

Most nitrogen in waterways comes from urine, through animals urinating in or close to streams, or from dairy-shed effluent. Phosphorus, spread as fertiliser on paddocks cleared of forest, washes into waterways along with soil, so levels of the element are closely linked to sediment load.

Accelerated erosion, mostly from deforestation, has increased the amount of sediment in our rivers. Sediment is not such a problem for humans – although it can obscure underwater hazards such as rocks and logs – but it can upset a waterway’s natural ecosystem by interfering with plant and animal life.

As for the amoeba that scared me in the 1970s, Naegleria fowleri is a single-celled animal that lives in the soil surrounding natural hot pools and is sometimes found in the water. Nancy Swarbrick writes on Te Ara that “diving into such pools or even immersing the head can force water up the nose, allowing the amoeba to invade the brain”.

There were nine fatal cases of amoebic meningitis between 1968 and 2000; commercial hot pools now filter water to keep it safe.


About Rebecca Priestley

I have a PhD in the history & philosophy of science and I write about science and science history. I live in New Zealand.
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